mtree as a particle effect
So, I am back with another Mtree answer, I guess.
I will tell you my way of doing it, may not be the best one.
What I do, is I first join the leaves with the trunk.
How?
Well, First, I take the leaves and in edit mode, I assign all their vertices to a single Vertex group. Why I do this? I will answer in a moment.
Then, I join the leaves object to the trunk (Ctrl + J). You will see that the leaves are gone. But why? Well, the leaves object is just vertices, emitting hair. So, by joining them in the trunk, they lose their modifiers. And particle systems are modifiers, so now the particle system is gone. That's why, I go to the particle system settings, add a new particle system, from the drop down menu I choose &quot;leaf&quot;(that's the one that was on the leaves object) and then, down there, at Vertex group section, I set the density to be that one group I created before.
Now, I can add the hair system on the terrain object. If a hair instance object has his own hair, it will  be displayed, but only is its from type &quot;Path&quot;(default streams of hair), it will not if its an object. So, that's why, now I convert the trees into meshes(before that I hide the Leaf particle system so my pc doesn't crash). So, now its working! But, there is one thing you will notice- if the hair system on the terrain had the trees smaller, than their original size, they will look like bushes- because the leaves didn't change in size. So, now we have to it. We go to the particle system of the plane and we see what is the scale of the instance object- if it was 1:1, it will be exactly 0.25. So, if yours is smaller/bigger, we need to find how much. If we divide the 1:1 scale(.25) by the current scale(lets say its .05), we get 5. So, this means that the trees are 5 times smaller. If you get a value of, say .5, that means that you actually scaled them up(in the case of .5 its twice bigger). So, now we have to change the scale of the leaves with that ratio. So, if we get that the trees are 5 times smaller, then we also scale the leaves down 5 times. We do this from the hair particle system settings, same as from where we got the tree scale.
And now, you should be set.